Chikungunya virus (CHIKV) is a mosquito-transmitted Alphavirus belonging to family Togaviridae [1,2]. It was isolated for the first time from a Tanzanian outbreak in 1952 [3]. It is responsible for an acute infection of abrupt onset, characterized by high fever, arthralgia, myalgia, headache and rash [4,5]. Poly-arthralgia, the pathognomonic sign of the disease, is very painful. Symptoms are generally self-limiting and last 1 to 10 days. However, arthralgia or arthritic symptoms may persist for months or years. In some patients, minor hemorrhagic signs such as epistaxis or gingivorrhagia have also been described.
CHIKV is geographically distributed in Africa, India and South East Asia. In Africa, the virus is maintained through a sylvatic transmission cycle between wild primates and mosquitoes such as Aedes luteocephalus, Ae. furcifer or Ae. taylori [4]. In Asia, CHIKV is mainly transmitted from human to human by Ae. aegypti and to a lesser extent by Ae. albopictus through an urban transmission cycle. Since the 1952 Tanzania outbreak, CHIKV has caused outbreaks in East Africa (Tanzania, Uganda), in Austral Africa (Zimbabwe, South Africa), in West Africa (Senegal, Nigeria) and in Central Africa (Central African Republic, Democratic Republic of the Congo) [4]. The most recent epidemic re-emergence was documented in 1999-2000 in Kinshasa, where an estimated 50,000 persons were infected [6]. Since the first documented Asian outbreak in 1958 in Bangkok, Thailand, outbreaks have been documented in Thailand, Cambodia, Vietnam, Laos, Myanmar, Malasia, Philippines and Indonesia [4,5]. The most recent epidemic re-emergence was documented in 2001-2003 in Java after 20 years [7]. Either in Africa or Asia, the re-emergence was unpredictable, with intervals of 7-8 years to 20 years between consecutive epidemics.
Since the end of 2004, Chikungunya virus (CHIKV) has emerged in the islands of the south-western Indian Ocean. Between January and March 2005, more than 5,000 cases were reported in Comoros. Later in 2005, the virus has circulated in the other islands, i.e Mayotte, Seychelles, Réunion and Mauritius. Starting in December 2005, the rainy season gave rise to a renewed epidemic circulation of the virus. Between January 1st and Mar. 1, 2006, 2,553, 3,471, and 4,650 cases have been reported in Mauritius, Mayotte and Seychelles (Mar. 12, 2006). The most affected island is Reunion with an estimated 212,000 cases until Mar. 12, 2006 (total population: 770,000). More recently, circulation of the virus has been documented in Madagascar.
In Reunion Island, the first documented cases were patients coming 1 ng back from Comoros in March 2005. More than 3,000 cases were reported from March to June. The transmission was limited during the winter season of the southern hemisphere and a major upsurge has been observed since mid-December, with an estimated 210,000 cases between January and March 2006 [8]. Since March 2005, 85 patients with a confirmed CHIKV infection have developed severe clinical signs (meningoencephalitis or fulminant hepatitis) which justified hospitalization in an intensive care unit. Several cases of meningo-encephalitis and major algic syndrome have been associated with vertical transmission of the virus 9.
To date, two CHIKV complete nucleotide sequences have been determined, for the strains Ross (accession no: AF490259) and S27 [9], both isolated from patients during the 1952 Tanzania outbreak. Another complete nucleotide sequence has been determined for a strain isolated in Ae. furcifer during the Senegal 1983 outbreak (accession no AY726732). Khan and coworkers [9] showed that the S27 genome was similar in its structure to that of other alphaviruses and that O'nyong-nyong virus (ONN) was the closest relative to CHIKV. In addition, phylogenetic analyses based on partial E1 sequences from African and Asian isolates revealed the existence of three distinct CHIKV phylogroups, one containing all isolates from West Africa, one containing isolates from Asia, and one corresponding to Eastern, Central and Southern African isolates [10]. Strains isolated in 1999-2000 in the Democratic Republic of the Congo belonged to the latter phylogroup [6].